This invention relates to centrifugal fans and, more particularly, to a centrifugal fan having a mechanism for varying the effective width of the fan's blades while the fan is in operation.
Many modulation schemes exist by which the regulation of centrifugal fan operation can be accomplished so as to reduce fan power consumption when load conditions permit. Among such fan modulation schemes are systems predicated on the use of (1) discharge dampers, (2) inlet guide vanes, (3) eddy-current clutches, (4) variable speed belt drives, (5) AC frequency inverters, (6) variable pitch fan blades and (7) hubs, disposed radially interior of the blade set, which are moveable within a fan wheel.
All of the above in one way or another relate to the control of a centrifugal fan so as to minimize power consumption by the fan motor. Several deal with the varying of fan speed in accordance with load conditions while others allow for fan operation at constant speed but at reduced load.
The selection of a particular fan modulation scheme depends upon many factors. Among these factors are system size, fan and system operating conditions, load distribution, fan type (i.e., forward curved, backwardly inclined or air foil blades) as well as maintenance requirements, space available for fan installation and, often most importantly, noise and cost.
The overall goal of fan modulation is to deliver only the required volume of air based upon local demand conditions at the lowest energy and initial investment costs. The favored and most economical fan modulation scheme, when first cost is a primary consideration, involves the use of inlet guide vanes on air handlers equipped with forward curved blades. Inlet guide vane mechanisms, as best exemplified by U.S. Pat. No. 4,177,007 which is assigned to the assignee of the present invention, are relatively simple yet rugged and cost-efficient apparatus by which fan modulation can be accomplished.
Inlet guide vanes modulate fan load by imparting a spin to the air delivered to the fan wheel in the direction of fan wheel rotation. The effect of this spin or pre-swirl is to cause the unloading of the fan blades which decreases the volume of the air delivered by the fan which, in turn, decreases the horsepower required to drive the fan wheel. While currently the fan modulation method of choice, along with dramatically more expensive variable speed inverter drives, inlet guide vanes do have drawbacks which detract from their efficiency and attractiveness for use.
A primary disadvantage in the use of inlet guide vanes relates to their disposition near or in the inlet of a centrifugal fan where they act as an impediment to airflow at peak load conditions. Conversely, at low load conditions many inlet guide vane mechanisms are "leaky" and allow for the passage of a significant amount of unneeded air into the fan housing. This additional air only adds to the load on the fan and does not serve any purpose with respect to building climate control.
Further, inlet guide vanes can, under some circumstances, be relatively noisy both with respect to their mechanical operation and in their interaction with air flowing through the fan inlet. Noise can be an extremely critical factor in many fan installations, particularly to the extent the noise might be communicated into a work or office space adjacent to or near the fan.
One approach to fan modulation, if successfully implemented with a sufficient degree of reliability and efficiency as has not yet heretofore been the case, relates to varying the effective width of the blades of a centrifugal fan wheel while the fan is in operation. By varying effective blade width, essentially all of the advantages of inlet guide vanes result while many of the disadvantages are negated. The superior power unloading characteristics of variable blade width fans is significant as, potentially, is their quietness, all at a cost competitive with inlet guide vanes and dramatically less than variable speed drives.
The primary reason for the lack of a commercially viable, highly efficient centrifugal fan blade width varying arrangement relates to the high operating speeds and centrifugal forces which exist with respect to fan operation and the need to have the modulation apparatus rotate with the fan wheel to achieve both the efficiency and quietness goals. Only one arrangement relating to variable width fan blades is known where the apparatus by which the effective blade width is changed rotates with the fan wheel. That arrangement, illustrated in U.S. Pat. No. 3,019,963, relates to a blower for use with gases having high dust content.
The modulation apparatus of the latter device includes a very substantial moveable disc of box like construction the displacement of which changes the admission of the "intensive gas-dust mixture". The use of the disc is a feature which can be provided "in addition to" a "guiding device" where the guiding device consists of airflow guiding vanes disposed in the inlet of the fan. Such construction is not appropriate to a centrifugal fan used in air delivery systems for building ventilation purposes where fan noise is an extremely critical factor. Further, because the "unused" portion of the fan blades remain exposed to air within the fan housing, the unused portion of the blades and the air within the housing do interact to the detriment of fan efficiency and quietness.
Other arrangements by which the effective width of centrifugal fan blades have been modulated include the use of shroud mechanisms, as is illustrated in Patentschrift 364732 and U.S. Pat. No. 4,135,850. Still other related mechanisms employ moveable members disposed interior of the fan wheel which, when actuated, block off a portion of the fan wheel from airflow through the fan inlet. Exemplary in this regard are U.S. Pat. Nos. 4,808,068 and 4,929,150. Virtually all such arrangements, because they do not rotate with the fan wheel, leave the "blocked off" portions of the fan blades exposed to air flowing within the housing which adversely affects fan efficiency and noise characteristics.
The need therefore continues to exist for a centrifugal fan modulation arrangement, operable while the fan is in operation, by which the effective width of the blades of the fan is varied in accordance with airflow demand and which achieves efficiencies and quietness levels not heretofore attained through the isolation of the unused portions of the fan blades and the prevention of their interaction with air flowing through the fan housing.